A. N. Maksimova

Magnetization reversal processes in layered high-temperature superconductors with ferromagnetic impurities

The self-consistent interaction of a vortex system of a high-temperature superconductor and ferromagnetic impurities, including single impurities and their clusters, has been considered in the model of a layered high-temperature superconductor. For different temperatures and concentrations of ferromagnetic impurities, the magnetization reversal loops have been calculated by the Monte Carlo method taking into account an ensemble of ferromagnetic particles with different orientations of their easy magnetization axes with respect to the direction of an external magnetic field and for different magnetic anisotropy energies. It has been demonstrated that there is a nonlinear interaction of the high-temperature superconductor with ferromagnetic impurities, in which the initially thermodynamically reversible character of the magnetization reversal of the ferromagnetic ensemble can become irreversible. For a periodic lattice of clusters of ferromagnetic impurities, the magnetization curves of the high-temperature superconductor have been calculated for different sizes and configurations of the clusters. It has been revealed that, when extended defects are oriented parallel to the direction of the entrance of vortices in the sample, the length of the defects does not affect the remanent magnetization. It has been shown that the inclusion of the interaction between the magnetic moments inside the impurity cluster leads to a decrease in the magnetization reversal loop, the coercivity, and, accordingly, the energy loss due to magnetization reversal.

Nonlinear interaction of a ferromagnet with a high-temperature superconductor

The interaction of an Abrikosov vertex with a ferromagnetic substrate is taken into account in the model of a layered high-temperature superconductor (HTSC). The magnetization reversal loops are calculated by the Monte Carlo method for various values of the magnetic moment of the substrate and at various temperatures. The nonlinearity of the interaction of the superconductor with the ferromagnet is demonstrated. The magnetization of HTSC films on magnetic and nonmagnetic substrates is measured. It is found that the ferromagnetism of the substrate strongly affects the shape and magnitude of the magnetization of the HTSC-substrate composite. Experimental data are found to correlate with the results of calculations.

Nonlinear effects at the remagnetization of layered high-temperature superconductors with ferromagnetic impurities under the action of a current and an external magnetic field

The processes of remagnetization by the self-field of a transport current in a high-temperature superconductor sample with ferromagnetic particles as defects in the presence of an external magnetic field have been calculated by the continuous Monte Carlo method. A new effect has been detected in the form of a nonlinear S-shaped character of current-voltage characteristics of the superconductor-ferromagnet in the external magnetic field. It has been shown that the S-shaped feature of the current-voltage characteristic is due to change in the effective interaction of magnetic moments of impurity particles with the vortex system of the superconductor induced by the local remagnetization of ferromagnetic impurities under the action of the self-field of the transport current and by the motion of the magnetic domain and the vortex density domain. The temperature, magnetic field, and imperfection parameters of the sample at which nonlinear effects are manifested have been analyzed. The conditions for the possible generation of radiation in the region of nonlinearity of the current-voltage characteristic have been obtained. The H-T phase diagram of the existence of the instability of the current-voltage characteristic has been plotted.

Effect of Anisotropy on the Transport Properties of Layered High-temperature Superconductors with Extended Magnetic and Nonmagnetic Defects

Transport properties of anisotropic superconductor with point, columnar and columnar tilted defects have been analyzed using Monte-Carlo method. It was shown that for columnar tilted defects the critical current dependence on anisotropy γ weakens as γ increases and vanishes at certain γ. The S-type nonlinearity of current-voltage characteristics has been shown for three-dimensional vortex system in presence of ferromagnetic defects.

Magnetization of Two-dimensional Layered HTSCs with Defects

Calculations of behaviour of magnetization of type-II superconductors in external magnetic fields have been made by using the Monte-Carlo method within the limits of the model of layered HTSC. Magnetization curves have been acquired for samples of the most common materials. The influence of concentration and geometry of defects on magnetization processes has been investigated. Residual magnetization dependency on the concentration of defects and characteristic lengths of superconductors has been estimated.

Magnetization and transport characteristics of layered high-temperature superconductors with different anisotropy parameters

The magnetization of a layered high-temperature superconductor with different anisotropy parameters has been calculated using the Monte Carlo method in the framework of a modified three-dimensional Lawrence–Doniach model with actual boundary conditions. The penetration of a magnetic flux into a bulk sample from the boundary has been simulated, and the curves of magnetization reversal of a high-temperature superconductor by an external magnetic field have been calculated for different anisotropy parameters γ and types of defects in the sample. It has been found that there are significant differences in the magnetization curves and transport properties of superconductors with different anisotropy parameters γ. The influence of tilted columnar defects on the critical current has been analyzed. A decreasing dependence of the critical current on the tilt angle of defects with respect to the c axis has been obtained. It has been shown that, as the anisotropy parameter increases, this dependence weakens and, for a specific value of γ, disappears. An explanation of the mechanism responsible for the disappearance of the dependence has been proposed.

The Magnetization Processes in Layered High-Temperature Superconductors: The Effect of Anisotropy

The magnetization of layered high-temperature superconductors (HTSCs) has been studied by using the Monte Carlo method. Typical parameters of promising HTSCs such as BSCCO, MgB2, and iron-based superconductors have been used for simulations, and the magnetization reversal loops were obtained. Anisotropy dependence of magnetic properties was analyzed. The ferromagnetic nanorods of arbitrary radius and constant magnetization were considered as pinning centers, and the pinning properties of extended ferromagnetic defects were analyzed. The analytic solution for the interaction of the vortex line and the ferromagnetic nanorod was obtained in the Lawrence-Doniach model. The simulations were done for both ferromagnetic and nonmagnetic extended defects.

Domain of a magnetic flux in superconductors with ferromagnetic pinning centers

The processes of magnetization reversal by the transport current self-field of a high-temperature superconductor sample with ferromagnetic particles as defects in the presence of an external magnetic field and current-voltage characteristics of the system have been calculated using the Monte Carlo method. It has been found that an S-shaped feature is observed in the current-voltage characteristics due to a change in the effective interaction of the magnetic moments of impurity particles with a vortex system of the superconductor, which is induced by the magnetization reversal of ferromagnetic impurities under the influence of the local magnetic field of the vortex lattice. It has been demonstrated that, in the vicinity of the instability, there is the motion of a wave of magnetization reversal of magnetic defects (magnetic domain) and waves of annihilation of Abrikosov vortices (vortex density domains).